1. Field of the Invention
The invention relates to the processing of C.sub.8 aromatic hydrocarbons to recover a desired isomer of xylene, and in particular to the recovery of o-xylene and p-xylene.
2. Discussion of the Background
Distillation columns are used to separate selected components from multicomponent streams by fractional distillation. It has been known that the presence of a catalyst in a distillation column can be useful in the production of various aliphatic hydrocarbons. This is particularly true of processing lighter aliphatic hydrocarbons.
U.S. Pat. No. 2,403,672 of Matuszak shows separating isobutylene from butene-1 in a combined isomerization reactor and fractional distillation column.
U.S. Pat. No. 3,634,534 of Haunschild discloses a process carried out in distillation columns having catalyst disposed in selected downcomers. The disclosure shows processing a C.sub.5 refinery stream. It is described as being advantageously applied to the separation of nontertiary olefins from tertiary olefins.
My U.S. Pat. No. 5,277,847 of Gentry et al. discloses improved catalyst-downcomer-tray assemblies for vapor-liquid contact towers.
In contrast to technology such as shown in the patents of Matuszak and Haunschild, the production of heavier hydrocarbons and the isolation of desired isomers thereof has involved the use of structure and processes that are more complex and, importantly, more expensive. This has been particularly true in the recovery of aromatic hydrocarbons. Specific reference is made to processing streams having significant content of C.sub.8 aromatic hydrocarbons and isolating o-xylene therefrom. Such streams may contain significant amounts of ethylbenzene, but not necessarily.
The production of specific xylene isomers is an important petrochemical process. Large quantities of p-xylene are consumed in processes leading to the production of polyesters used in clothing manufacture. O-xylene is used as a raw material in the production of phthalic anhydride which, in turn, is used for plasticizers, for alkyd resins, and for polyesters. Of the isomers of xylene, it is the p-xylene that is generally considered to be the most valuable and desired product. Not surprisingly, a process that values the recovery of o-xylene is also likely to be adapted to recovering p-xylene.
It is known to separate o-xylene from mixed xylenes by conventional distillation. The stream that is thus depleted of o-xylene typically is routed to a separate unit to separate and recover p-xylene. In the alternative, the p-xylene may be removed before the xylene distillation step. The result is a stream of C.sub.8 aromatic hydrocarbons that is deficient in o-xylene and p-xylene, and richer in ethylbenzene and m-xylene.
In the current state of the art, there are two main classes of xylene isomerization reactions. In one class, all four of the C.sub.8 aromatic isomers are isomerized toward an equilibrium mixture. In the other class, only the xylenes are isomerized toward an equilibrium mixture; the ethylbenzene is converted into benzene. Both classes of reaction yield approximately an equilibrium ratio of the xylene isomers, regardless of their ratio in the feed stream.
U.S. Pat. No. 3,770,841 of Meyers discloses a process for the recovery of p-xylene in which o-xylene or m-xylene or both may also be recovered as desired. The process includes subjecting a feed stream to an initial separation step, which may be a chromatographic or adsorption step, to recover p-xylene and remove ethylbenzene. Then in a series of fractional distillation columns, m-xylene and o-xylene are recovered as overheads and C.sub.9+ is drawn off as a bottom. One of the overhead streams, which contains o-xylene and m-xylene, is passed to a reactor, where it undergoes catalytic isomerization to convert at least a portion of the stream to additional p-xylene. Preferably the isomerization is a low temperature isomerization carried out in the presence of a toluene diluent. The output of the reactor is fed to a distillation column, where the toluene is recovered along with any benzene that may be present. The bottoms are fed to an additional separation stage, which may be like the first, for the recovery of p-xylene. The output of that stage is fed to the above-mentioned series of fractional distillation columns.
U.S. Pat. Nos. 4,697,039 and 4,783,568 of Schmidt disclose processes for the production of a desired xylene isomer, preferably p-xylene. The feed stream enters at 1 and optionally at 33. Following a stage 2 for the separation of p-xylene, the stream passes through a xylene isomerization zone at 6, optional fractional distillation at 12, and a transalkylation zone 14 where dealkylation and isomerization occur. The output of zone 14 is fed to a series of fractional distillation columns where C.sub.6, C.sub.7, C.sub.8, and C.sub.9 streams, respectively, are drawn off as overhead. Column 29 is called a xylene column. In the embodiment shown in the drawing, the overhead of this column is fed into a p-xylene separation zone in the form of p-xylene recovery unit 2 to allow the recovery of p-xylene. In a disclosed variation (e.g., '039 patent, column 9, lines 48-51), the raffinate stream of the xylene separation zone is further fractionated to produce a stream rich in o-xylene.
Yet another example of prior art apparatus and process is shown in FIG. 1 herein. A feed stream 5 containing mixed xylenes is introduced typically at a location between the middle portion and the top of a xylene splitter column 10. As typical, the mixed xylene stream contains ethylbenzene. The stream may contain heavier and lighter hydrocarbons. The preferred location of the feed may change depending on its composition.
The stream enters a xylene splitter zone shown in the drawing in the form of fractional distillation towers 12 and 14. For economic reasons it is typical for this zone to be fabricated as two separate towers, as shown. If desired, they could be fabricated as one tower without significant alteration of their function. For purposes of the present disclosure, the two towers 12 and 14 collectively will be treated functionally as a single column 10. A typical xylene splitter may contain between seventy-five and one hundred fifty trays.
O-xylene is drawn off in the bottoms of the xylene splitter 10, together with C.sub.9+. Typically more than ninety-five percent of the p-xylene and m-xylene will have been removed. If desired, these bottoms may be subjected to further distillation in an additional column, not shown in the drawing, for the recovery of the o-xylene from the C.sub.9+.
The overhead of the xylene splitter 10, being enriched in p-xylene, is fed to a known p-xylene recovery unit 16, which may be of a type discussed in the patents noted above. Therein, p-xylene is recovered as a valuable product. Typically such units are sensitive to the p-xylene concentration in the feed. In this regard, the separation of p-xylene is aided by the prior removal of most of the o-xylene.
The output of the p-xylene recovery unit 16 will have been partially depleted of o-xylene and p-xylene. It is fed to an isomerization reactor 18 which contains a catalyst. Therein, the xylenes and ethylbenzene in the stream undergo catalytic isomerization to produce a stream that is closer to an equilibrium state for xylene isomers. One result of the isomerization step is that the output of the isomerization reactor will be enriched in p-xylene and o-xylene as compared with the feed to the isomerization reactor.
The output of the isomerization reactor is fed to a flash drum 20 for the removal of any H.sub.2 or other gases that might have resulted from the cracking of ethylbenzene. The stream then is fed to a fractional distillation column 22 to distill overhead any benzene that may be present. Benzene is expected to be produced in small amounts by the cracking of ethylbenzene or xylenes during the isomerization reaction. The bottoms of column 22 are fed to another fractional distillation column 24, where toluene is recovered as an overhead. The bottoms of column 24 are mixed with the fresh feed 5 to the system and reintroduced to the xylene splitter 10.
The economic viability of any process for the production of xylenes is dependent on several factors. Among these are the total yield of the desired xylene isomer or isomers; the initial capital cost of the equipment such as columns, reactors and piping and the catalyst necessary for operating the process; and the cost of utilities. With specific reference to the recovery of o-xylene, the profitability of fabricating or operating a xylene splitter is further governed by the benefit that upstream o-xylene recovery confers on the p-xylene recovery step and by the relationship of the incremental energy cost of operating the xylene splitter to the price difference between o-xylene and mixed xylenes.
Given these known constraints and goals, the prior art discussed above nonetheless is characterized by the comparatively high capital cost of plural distillation columns disposed in a loop with an isomerization reactor and, typically, a p-xylene recovery unit. O-xylene recovery by fractional distillation, isomerization of the xylenes in the stream toward an equilibrium state, and p-xylene separation occur in discrete steps, sequentially, in separate units.